Radio frequency leakage detection system for CATV system

ABSTRACT

A communication circuit carries at least one signal including synchronizing intervals by which output of the signal by first apparatus coupled to the communication circuit is to be synchronized. A communication method includes selecting and modulating at least a portion of the duration of at least some of the synchronizing intervals. A modulator is provided for selecting and modulating at least a portion of the duration of at least some of the synchronizing intervals. Another communication circuit carries at least a first signal including frequencies in at least a first frequency band. A method of determining whether the first signal is escaping from the circuit includes transporting along the circuit apparatus adapted for receiving the first signal, detecting whether a received signal includes frequencies in the first frequency band, and producing an indication that the received signal includes frequencies in the first frequency band. A detector is provided for transporting along the circuit for determining whether the first signal is escaping from the circuit. The detector is adapted for receiving the first signal, detecting whether a received signal includes frequencies in the first frequency band, and producing an indication that the received signal includes frequencies in the first frequency band.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing dates of U.S. Ser. No.60/145,867 filed Jul. 28, 1999, and U.S. Ser. No. 60/170,267 filed Dec.11, 1999.

FIELD OF THE INVENTION

This invention relates to the detection of RF leakage from communicationcircuits. It is disclosed in the context of an RF leakage detectionsystem for a CATV system, but is believed to be useful in otherapplications as well.

BACKGROUND OF THE INVENTION

Techniques for the detection of RF leakage from CATV systems are known.There are, for example, the systems described in U.S. Pat. Nos.5,608,428; 5,294,937; 4,520,508; 4,491,968; 4,413,229; 4,237,486;4,072,899; 3,711,767. There are also the systems described in U.S. Pat.Nos. 5,585,842; 5,493,210; 5,321,089; 5,210,498; 5,210,497; 4,962,358;4,814,694; 4,810,961; 4,775,839; 4,731,586; 4,670,789; 4,609,866;3,882,287; 3,684,823; 3,368,031; 3,345,560; 3,155,897; and, 2,291,533.There is also the disclosure of U.S. Ser. No. 08/690,122, filed Jul. 31,1996. There is also the disclosure of Archer S. Taylor, Characterizationof Cable TV Networks as the Transmission Media for Data, IEEE Journal onSelected Areas in Communications, vol. SAC-3, no. 2, March 1985, pp.255-265. The disclosures of all of these references are herebyincorporated herein by reference. This listing is not intended as arepresentation that the disclosures listed above are pertinent, or thata thorough search of the prior art has been conducted, or that no morepertinent disclosures than those listed above exist, and no suchrepresentations should be inferred.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention, a communication circuitcarries at least one signal including synchronizing intervals by whichoutput of the signal by first apparatus coupled to the communicationcircuit is to be synchronized. A communication method includes selectingand modulating at least a portion of the duration of at least some ofthe synchronizing intervals.

Illustratively according to this aspect of the invention, the methodfurther includes transporting along the circuit second apparatus adaptedfor receiving the signal, detecting the modulation, and producing anindication that the modulation has been detected for determining whetherthe signal is escaping from the circuit.

Further illustratively according to this aspect of the invention,selecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes modulating the amplitude ofat least a portion of the duration of at least some of the synchronizingintervals.

Additionally illustratively according to this aspect of the invention,selecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes sinusoidally modulating atleast a portion of the duration of at least some of the synchronizingintervals.

Illustratively according to this aspect of the invention, selecting andmodulating at least a portion of the duration of at least some of thesynchronizing intervals includes generating a first sinusoid having afirst frequency, generating a second sinusoid having a second frequency,frequency modulating the first sinusoid with the second sinusoid toproduce a frequency modulated signal, and modulating at least a portionof the duration of at least some of the synchronizing intervals with thefrequency modulated signal.

Further illustratively according to this aspect of the invention,detecting the modulation includes detecting the frequency modulatedsignal.

Alternatively illustratively according to this aspect of the invention,detecting the modulation includes detecting the second sinusoid.

Alternatively illustratively according to this aspect of the invention,detecting the modulation includes detecting the first sinusoid.

Additionally illustratively according to this aspect of the invention,selecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes modulating the amplitude ofat least a portion of the duration of a relatively constant portion ofthe synchronizing intervals.

Illustratively according to this aspect of the invention, selecting andmodulating at least a portion of the duration of at least some of thesynchronizing intervals includes sinusoidally modulating at least aportion of the duration of substantially all of the synchronizingintervals.

Further illustratively according to this aspect of the invention,selecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes modulating the amplitude ofsubstantially all of the duration of at least a portion of thesynchronizing intervals.

Additionally illustratively according to this aspect of the invention,selecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes sinusoidally modulatingsubstantially all of the duration of at least a portion of thesynchronizing intervals.

According to another aspect of the invention, a communication circuitcarries at least one signal including synchronizing intervals by whichoutput of the signal by first apparatus coupled to the communicationcircuit is to be synchronized. A modulator is provided for selecting andmodulating at least a portion of the duration of at least some of thesynchronizing intervals.

Illustratively according to this aspect of the invention, the apparatusfurther includes a detector for transportation along the circuit. Thedetector is adapted for receiving the signal, detecting the modulation,and producing an indication that the modulation has been detected fordetermining whether the signal is escaping from the circuit.

Further illustratively according to this aspect of the invention, themodulator includes a modulator for selecting and modulating theamplitude of at least a portion of the duration of at least some of thesynchronizing intervals.

Additionally illustratively according to this aspect of the invention,the modulator includes a modulator for sinusoidally modulating at leasta portion of the duration of at least some of the synchronizingintervals.

Illustratively according to this aspect of the invention, the modulatorincludes a modulator for generating a first sinusoid having a firstfrequency, generating a second sinusoid having a second frequency,frequency modulating the first sinusoid with the second sinusoid toproduce a frequency modulated signal, and modulating at least a portionof the duration of at least some of the synchronizing intervals with thefrequency modulated signal.

Further illustratively according to this aspect of the invention, thedetector includes a detector for detecting the frequency modulatedsignal.

Alternatively illustratively according to this aspect of the invention,the detector includes a detector for detecting the second sinusoid.

Alternatively illustratively according to this aspect of the invention,the detector includes a detector for detecting the first sinusoid.

Additionally illustratively according to this aspect of the invention,the modulator includes a modulator for modulating the amplitude of atleast a portion of the duration of a relatively constant portion of thesynchronizing intervals.

Illustratively according to this aspect of the invention, the modulatorincludes a modulator for sinusoidally modulating at least a portion ofthe duration of substantially all of the synchronizing intervals.

Further illustratively according to this aspect of the invention, themodulator includes a modulator for modulating the amplitude ofsubstantially all of the duration of at least a portion of thesynchronizing intervals.

Additionally illustratively according to this aspect of the invention,the modulator includes a modulator for sinusoidally modulatingsubstantially all of the duration of at least a portion of thesynchronizing intervals.

According to yet another aspect of the invention, a communicationcircuit carries at least a first signal including frequencies in atleast a first frequency band. A method of determining whether the firstsignal is escaping from the circuit includes transporting along thecircuit apparatus adapted for receiving the first signal, detectingwhether a received signal includes frequencies in the first frequencyband, and producing an indication that the received signal includesfrequencies in the first frequency band.

Illustratively according to this aspect of the invention, transportingapparatus adapted for receiving the first signal includes transportingapparatus through an environment including at least a second signalincluding frequencies in at least a second frequency band not includingfrequencies in the first frequency band.

Further illustratively according to this aspect of the invention, themethod includes detecting whether a received signal includes frequenciesin the second frequency band.

Additionally illustratively according to this aspect of the invention,the method includes comparing the magnitude of received frequencies inthe first frequency band to the magnitude of received frequencies in thesecond frequency band. Producing an indication that the received signalincludes frequencies in the first frequency band includes producing anindication of the relationship of the magnitude of received frequenciesin the first frequency band to the magnitude of received frequencies inthe second frequency band.

Illustratively according to this aspect of the invention, comparing themagnitude of received frequencies in the first frequency band to themagnitude of received frequencies in the second frequency band andproducing an indication of the relationship of the magnitude of receivedfrequencies in the first frequency band to the magnitude of receivedfrequencies in the second frequency band together include comparing themagnitude of received frequencies in the first frequency band to a firstdesired threshold and producing a third signal based upon thiscomparison, comparing the magnitude of received frequencies in thesecond frequency band to a second desired threshold and producing afourth signal based upon this comparison, and combining the third andfourth signals to produce the indication of the relationship of themagnitude of received frequencies in the first frequency band to themagnitude of received frequencies in the second frequency band.

According to yet another aspect of the invention, a detector is providedfor transporting along the circuit for determining whether the firstsignal is escaping from the circuit. The detector is adapted forreceiving the first signal, detecting whether a received signal includesfrequencies in the first frequency band, and producing an indicationthat the received signal includes frequencies in the first frequencyband.

Illustratively according to this aspect of the invention, the detectoris adapted for receiving a second signal including frequencies in atleast a second frequency band not including frequencies in the firstfrequency band.

Illustratively according to this aspect of the invention, the detectoris further adapted for comparing the magnitude of received frequenciesin the first frequency band to the magnitude of received frequencies inthe second frequency band, and producing an indication of therelationship of the magnitude of received frequencies in the firstfrequency band to the magnitude of received frequencies in the secondfrequency.

Illustratively according to this aspect of the invention, the detectorcompares the magnitude of received frequencies in the first frequencyband to the magnitude of received frequencies in the second frequencyband and produces an indication of the relationship of the magnitude ofreceived frequencies in the first frequency band to the magnitude ofreceived frequencies in the second frequency band by comparing themagnitude of received frequencies in the first frequency band to a firstdesired threshold and producing a third signal based upon thiscomparison, comparing the magnitude of received frequencies in thesecond frequency band to a second desired threshold and producing afourth signal based upon this comparison, and combining the third andfourth signals to produce the indication of the relationship of themagnitude of received frequencies in the first frequency band to themagnitude of received frequencies in the second frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdetailed description and accompanying drawings which illustrate theinvention. In the drawings:

FIG. 1 illustrates a block diagram of a system incorporating theinvention;

FIG. 2 illustrates a partly block and partly schematic diagram of adetail of the system illustrated in FIG. 1;

FIGS. 3 a-b illustrate waveforms useful in understanding the invention;

FIG. 4 illustrates a partly block and partly schematic diagram of adetail of another embodiment of the system;

FIG. 5 illustrates a partly block and partly schematic diagram of adetail of another system constructed according to the invention, andwaveforms useful in understanding the operation of the detail;

FIGS. 6 a-b illustrate partly block and partly schematic diagrams ofdetails of another system constructed according to the invention;

FIG. 7 illustrates a concept useful in understanding another embodimentof the invention; and,

FIG. 8 illustrates a partly block and partly schematic diagram ofanother embodiment of the invention.

DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIMENTS

Turning now to the drawings, it is known to place a slowly varyingmodulation, sometimes referred to as a “tag,” on a CATV channel's videocarrier or on an active CATV channel's video signal with the intent ofdetecting the tag, in order to determine whether there is leakage of thedetected channel, and hence, RF radiation generally, from the CATVsystem that is carrying the channel. Modulation frequencies proposed forsuch tags are in the very low frequency range, for example, 3 Hz to 35Hz, that television receivers' AGC circuits are designed to correct suchmodulation out of the received video. In this way, the subscribers whoare viewing the tagged channel on their receivers are unaware that thechannel is tagged, because their own receivers have removed the tagmodulation before the video signal on the tagged channel is processed.

A problem arises with certain set top terminals (STTs) which do not haveAGC circuits with bandwidths sufficient to remove tags in the desiredfrequency ranges. Such STTs have, for example, narrower AGC bandwidthsin, for example, the 3 Hz to <7 Hz range, so that if a CATV system tagsa channel with, for example, a 20 Hz tag, the 20 Hz modulation is notcorrected out of the signal processed by the STT. As a result, the 20 Hzmodulation remains in the video signal which is supplied to the receiverof a subscriber who is provided with such an STT. Such modulation canappear, for example, as an annoying 20 Hz modulation of the luminance ofthe picture being viewed by the subscriber. This modulation can besufficiently annoying to render the tagged channel incapable of beingviewed, and thus useless.

In certain embodiments of the invention, tag modulation appears only ina part of the video which is not displayed on subscribers' receivers,such as, for example, in the vertical blanking interval. It is known toplace other signals in the vertical blanking interval. However, it isnot believed to be known to place such a low frequency tag signal in thevertical blanking interval. Of course the vertical sync pulse occurs ata frequency of about 60 Hz. If a 20 Hz tag signal is used, for example,one cycle of the tag signal will occur every three vertical sync pulses.If a 3 Hz tag signal is used, one cycle of the tag signal will occurevery twenty vertical sync pulses. The tag modulation will affect theamplitude of the vertical sync pulses, but need not modulate theamplitude of the vertical sync pulses too deeply, for example, more thana few dB, in order to be detected by the tag detection equipment. Ofcourse, there must be some way for the STT or receiver to recover thevertical sync pulse, however much it is attenuated by tagging, in orderto produce an acceptable display on the receiver. Analysis of theamplitude of the vertical sync pulses over several fields, with aboutsixty fields occurring each second, makes determination of whether thevertical sync pulses being received are modulated at such a tagfrequency fairly straightforward. For example, a bandpass filter havinga narrow, sharply delineated passband centered on the tag frequency, canbe used to detect the tag and thus provide a determination whethervertical sync pulses in the video being received by a leakage detectorare in the tagged channel. If they are, the source of the leakage has,of course, been identified.

A CATV system 8 according to the present invention is illustrated inhighly schematic block diagram form in FIG. 1. Several program sources10, 12, 14, . . . 16, 18 such as satellite links, VCR's and the like ata head end 19 of the CATV system 8 are coupled to input ports ofrespective modulators 20, 22, 24, . . . 26, 28. The modulators modulatethe source signals into channels for placement on a CATV circuit 30. Theoutput ports of channel modulators 20, 22, 24, . . . 26 are coupledthrough respective power couplers 32, 34, 36, . . . 38 to circuit 30.The output port of an arbitrary one, 28, of the channel modulators,however, is selectively coupled by a switching network 124, 126 to aninput port of a variable attenuator, or amplitude modulator, 120. A low(here, subsonic) frequency oscillator 121 has an output port 123 onwhich appears a subsonic frequency signal in the range of, for example,3 Hz-35 Hz. This signal is coupled from port 123 to a control input port125 of amplitude modulator 120. The percentage modulation produced byamplitude modulator 120 in the output signal from modulator 28 is verylow, typically only about 3 dB or so of modulation of the source 18signal applied to circuit 30.

The detection of the oscillator 121 frequency in circuit 30 can best beunderstood by first assuming that circuit 30 contains a leak 156,illustrated as an antenna for purposes which will become clear, alongits length nearer head end 19 and a leak 158 along its length moreremote from head end 19. Leaks 156, 158 typically serve both asradiators for the egress of RF energy from CATV system 8 and asreceivers for the ingress of energy into CATV system 8. Thus it isimportant to identify and locate such leaks 156, 158 not only becausethe RF energy which would otherwise be available to subscribers 152, . .. 154 can escape the circuit 30 but also because RF signals of all kindsfrom all kinds of extraneous sources such as broadcast, two-way radioand so on are constantly impinging upon circuit 30.

Tag modulation is provided by amplitude modulator 120, for example, aPIN diode attenuator, during the vertical blanking interval by using thevertical sync pulses 122 to control two switches 124, 126 associatedwith the modulator 120. One, 124, of the switches steers the videosignal either through the modulator 120 or around it unmodulated and onconductor 128 out into the circuit 30, and the other, 126, of theswitches steers the tag-modulated signal back to the conductor 128 whichcarries the channel downstream in the circuit 30 to subscribers 52, . .. 54. Referring to FIG. 1, vertical sync 122 is detected 134, forexample, by a sync detector. The detected vertical sync 122 is coupledto, for example, the gate electrodes of four high-speed FET switches124-1, 124-2, 126-1 and 126-2. See FIG. 2. FETs 124-2 and 126-2 may beof opposite type to FETs 124-1 and 126-1, or the vertical sync 122 canbe inverted 136 before it is coupled to the gates of FETs 124-2 and126-2. The objective, of course, is to have the main current conductingpaths, drain-to-source or source-to-drain, of FETs 124-1 and 126-1conducting at all times other than during the vertical blankinginterval, and to have the main current conducting paths of FETs 124-2and 126-2 conducting only during the vertical blanking interval. In thisway, the modulator 120 is taken out of the circuit 30 at all times otherthan during the vertical blanking interval, and the bypass 138 is takenout of the circuit 30, and the modulator 120 placed in the circuit 30during the vertical blanking interval. The video signal, for example,modulated on a CATV channel carrier, is coupled to, for example, thedrains of FET switches 124-1 and 124-2. Terminals, for example, thedrains, of FET switches 126-1 and 126-2 are coupled to conductor 128.

What is put on the circuit 30 in the channel of which modulator 28 isthe modulator and channel carrier oscillator 160 is the channel carrieroscillator can best be understood by reference to FIGS. 3 a-b. In FIGS.3 a-b, for the purpose of clarity, what is between each pair of verticalsync pulses 122, namely, the 262 horizontal sync pulses and theapproximately 262½ lines of video, has been deleted. (Again, it must beremembered that all that passes through modulator 120 is the verticalsync pulses, owing to the switching of switches 124 and 126 beingcontrolled by the detected vertical sync pulses.) All that isillustrated is the train of vertical sync pulses 122 themselves. What isillustrated are thirty-three vertical sync pulses, roughly half asecond's worth. The depth of modulation of the vertical sync pulse 122train is somewhat exaggerated to illustrate the invention. In FIG. 3 a,the illustrated tag frequency is roughly 4 Hz and the modulating signalis a sinusoid. It should further be clear from this discussion that themodulating signal need not be an analog signal, such as the illustratedsinusoidal wave, at all. The modulation may, for example, be alternatingunmodulated and modulated vertical sync pulses 122, or some arbitrarynumber, for example, one, unmodulated vertical sync pulse 122, followedby some arbitrary number, for example, one, vertical sync pulse 122modulated to a depth of, for example, 3 dB. Such a modulation scheme isillustrated in FIG. 3 b. Again, the illustrated tag frequency is roughly4 Hz, and the depth of modulation of the vertical sync pulse 122 trainis somewhat exaggerated to illustrate the invention. Nor is it essentialto avoid frequencies up in the bandwidth of the television receiverstuned to the tagged channel. For example, a 2.5 KHz burst, a 500 KHzburst, or the like can be inserted into, for example, a 200 μsec. windowduring the vertical blanking interval in a location which does notaffect the ability of the vertical oscillators of television receiverstuned to the tagged channel to synchronize to the vertical sync pulse.

In another embodiment of the invention, illustrated in FIG. 4, themodulation during a portion of the vertical blanking interval isessentially complete. That is, the RF signal is substantially completelyattenuated during some portion of the vertical blanking interval. Aslong as a sufficient portion of the vertical sync pulse remainsunaffected to lock the vertical oscillators of television receiverstuned to the tagged channel, it makes little difference that someportion of the vertical blanking interval is substantially completelyattenuated. Since there are, in the NTSC format, approximately 60vertical blanking intervals every second, if the system operator wantsto tag at, for example, 10 Hz, some portion, for example, 200 μsec. orso, of roughly every sixth vertical blanking interval (10×6=60) can beattenuated by an amount permitting detection. If the system operatorwants to tag at 15 Hz, some portion of roughly every fourth verticalblanking interval (15×4=60) can be attenuated by an amount permittingdetection. If the system operator wants to tag at 20 Hz, some portion ofroughly every third vertical blanking interval (20×3=60) can beattenuated by an amount permitting detection. If the system operatorwants to tag at 30 Hz, some portion of every other vertical blankinginterval (30×2=60) can be attenuated by an amount permitting detection,and so on.

In another embodiment of the invention illustrated in FIG. 5, thecarrier f_(cn) of a channel n to be tagged, modulated by the informationto be transmitted in channel n, is coupled to one input port 200 of acombiner 202. The baseband video for channel n is coupled to a syncseparator 204. The sync for channel n is coupled from the sync separator204 to a pulse circuit 206 which generates a short duration, forexample, 50 μsec., pulse 208 during each, or every other, or everythird, or the like, vertical sync pulse 210 of the separated sync 212.Generating such a pulse 208 during every third vertical sync pulse 210has been demonstrated to be an effective strategy for several reasons,including the reason that power line frequency disturbances at 60 Hz donot appear to be as likely to alias the detection equipment. In anyevent, pulse 208 actuates a switch 214 between another signal source 216and another input port 218 of the signal combiner 202. What results atthe output port 220 of the signal combiner 202 is channel n plus a pulse208-duration burst of the output from signal source 216 during everysync pulse 210, or every other sync pulse 210, or every third verticalsync pulse 210, or the like. Signal source 216 may provide practicallyany desired signal, for example, a signal having a frequency f_(cT) nearthe lower end 222 of the channel n frequency band, perhaps modulated bya signal having a frequency f_(m) in the 1 KHz-10 KHz range. Thefrequency modulated carrier frequency f_(m)×f_(cT) lies below f_(n), thefrequency of the channel n carrier, and above the audio carrier 224 ofthe next adjacent lower channel (n−1).

In order to detect this signal to determine whether the detected leakageis coming from the tagged channel n, an instrument 226 such as theTrilithic, Inc., model SuperPlus leakage detection instrument ismodified as illustrated in FIGS. 6 a-b. Such an instrument 226 isgenerally as described in U.S. Pat. No. 5,608,428. In such an instrument226, the output signal from a receiver 228 is coupled to a switchedcapacitor filter 232 for further application to the instrument 226'ssquelch circuit to control the breaking of squelch. According to thepresent invention, an additional peak detection circuit 238 is coupledbetween an amplifier 236 in the receiver circuitry 228 and the switchedcapacitor filter 232. The peak detector 238 expands each pulse out, asillustrated at 240, and provides the train of expanded pulses 240 to theswitched capacitor filter 232. The output of the switched capacitorfilter 232 is then peak detected again, as illustrated at 242, as inU.S. Pat. No. 5,608,428, and compared 244 to a squelch threshold 246 tocontrol the breaking of squelch. Breaking squelch indicates that asignal having the center frequency f_(m) of the switched capacitorfilter 232 has been received, which in turn, is indicative that a leakof the tagged channel n has been received by the instrument 226.

Referring now to FIG. 7, in another embodiment of the invention whichmay be used independently of, or in conjunction with, other leakagedetection schemes, it first needs to be recognized that a considerableamount of the energy contained in baseband video appears in the DC to 4KHz or so frequency range, owing in large measure to the existence ofthe vertical sync pulses and other low frequency phenomena in basebandvideo. Noise from most sources encountered in the environment, on theother hand, tends to be more broadband. Thus, if a scheme is implementedto detect a signal in the environment surrounding a CATV system, andthat scheme detects signals having a preponderance of their energy atfrequencies greater than, for example, 5 KHz, the odds are greater thatthose signals are not baseband video, and thus, are not the result of achannel leaking from the CATV system. On the other hand, if the schemedetects signals having a preponderance of their energy at frequenciesless than some arbitrarily lower frequency, say 2 KHz or 3 KHz or 4 KHz,then the odds are greater that those signals are baseband video, andthus are the result of a channel leaking from the CATV system. One waysuch a system can be implemented is illustrated in FIG. 8.

In FIG. 8, CATV signals leaking 156, 158 from a CATV system 8 arereceived by an antenna 302 which is coupled to a receiver 304 includingan RF section and an IF section. An output of the IF section isdemodulated by tuning to, for example, the carrier frequency of one ofthe channels being carried by the CATV system 8, so that, if thatchannel is leaking from the CATV system 8 and is received by the antenna302, baseband video will be output from the IF section of the receiver304. The baseband video is detected by a detector circuit 306, bufferedby a buffer amplifier 308, and split into two signal paths, oneincluding a low pass filter (LPF) 310, and the other including a highpass filter (HPF) 312. Output signals from both LPF 310 and HPF 312 arepeak detected 314 and 316, respectively. Illustratively, the outputsignal from LPF 310 can be further processed, for example, byamplification by a display driver 318 or buffering or the like, and usedto generate, for example, a display 320 of the strength of a receivedCATV channel.

An output port of peak detector 314 is also coupled to an audiofrequency voltage controlled oscillator (VCO) 322 and to an input portof a comparator 324. An output port of peak detector 316 is coupled toan input port of a comparator 326. Additional input ports of comparators324, 326 are coupled to respective sources, for example, potentiometers,of DC voltages to permit the setting of thresholds above which theoperator of the equipment desires the compared signals to trigger outputsignals. Output ports of both comparators 324, 326 are coupled to inputports of an AND gate 328. An output port of AND gate 328 is coupled to acontrol input port of a squelch circuit 330 and to a control input portof driver 318. An output port of VCO 322 is coupled to another inputport of squelch circuit 330. An output port of squelch circuit 330 iscoupled to a loudspeaker 332.

Let a signal having a strength of, for example, 2 μV/m (about −113 dBmacross the output terminals of a 50Ω dipole) be received by a receiver304 having a 15 KHz IF bandwidth and tuned to, for example, 133 MHz. Thecontent of the 15 KHz passband is filtered by the LPF 310 and HPF 312having corner frequencies of, for example, 2 KHz and 4 KHz,respectively. The output of LPF 310 is peak detected 314 and used todrive 318 display 320 under the control of AND gate 328 to indicate thestrength of the received channel. The state of the output of comparator324 changes from 0 to 1 when the energy content of the received signalbelow 2 KHz exceeds comparator 324's set threshold. The state of theoutput of comparator 326 changes from 0 to 1 when the energy content ofthe received signal above 4 KHz does not exceed comparator 326's setthreshold. This combination of logical conditions produces at the outputof AND gate 328 a logic 1 condition, which, in turn, triggers thedisplay driver 318 and breaks squelch in circuit 330, permitting the VCO322 output to be transduced by loudspeaker 332. The output frequency ofVCO 322 changes in relation to the energy content in the DC to 2 KHzbandwidth, and thus provides an audible indication of the magnitude ofthe received CATV channel leakage.

The LPF does not need to have an upper corner frequency of 2 KHz, northe HPF a lower corner frequency of 4 KHz. These are illustrative. Forexample, a considerable amount of the energy contained in baseband videois contained in the vertical sync 122 at about 60 Hz. Thus, withappropriate adjustment of the DC voltages on the comparison input portsof comparators 324 and 326, an upper corner frequency of, for example,100 Hz, for LPF and a lower corner frequency of, for example, 1 KHz, forHPF could be useful.

1. In a communication circuit which carries at least one signalincluding synchronizing intervals by which output of the signal by firstapparatus coupled to the communication circuit is to be synchronized, acommunication method including selecting and modulating at least aportion of the duration of at least some of the synchronizing intervals.2. The method of claim 1 further including transporting along thecircuit second apparatus adapted for receiving the signal, detecting themodulation, and producing an indication that the modulation has beendetected.
 3. The method of claim 1 wherein selecting and modulating atleast a portion of the duration of at least some of the synchronizingintervals includes modulating the amplitude of at least a portion of theduration of at least some of the synchronizing intervals.
 4. The methodof claim 3 further including transporting along the circuit secondapparatus adapted for receiving the signal, detecting the modulation,and producing an indication that the modulation has been detected. 5.The method of claim 3 wherein selecting and modulating at least aportion of the duration of at least some of the synchronizing intervalsincludes sinusoidally modulating at least a portion of the duration ofat least some of the synchronizing intervals.
 6. The method of claim 5further including transporting along the circuit second apparatusadapted for receiving the signal, detecting the modulation, andproducing an indication that the modulation has been detected.
 7. Themethod of claim 5 wherein selecting and modulating at least a portion ofthe duration of at least some of the synchronizing intervals includesgenerating a first sinusoid having a first frequency, generating asecond sinusoid having a second frequency, frequency modulating thefirst sinusoid with the second sinusoid to produce a frequency modulatedsignal, and modulating at least a portion of the duration of at leastsome of the synchronizing intervals with the frequency modulated signal.8. The method of claim 7 further including transporting along thecircuit second apparatus adapted for receiving the signal, detecting themodulation, and producing an indication that the modulation has beendetected.
 9. The method of claim 8 wherein detecting the modulationincludes detecting the frequency modulated signal.
 10. The method ofclaim 8 wherein detecting the modulation includes detecting the secondsinusoid.
 11. The method of claim 8 wherein detecting the modulationincludes detecting the first sinusoid.
 12. The method of claim 3 whereinselecting and modulating at least a portion of the duration of at leastsome of the synchronizing intervals includes modulating the amplitude ofat least a portion of the duration of a relatively constant portion ofthe synchronizing intervals.
 13. The method of claim 12 furtherincluding transporting along the circuit second apparatus adapted forreceiving the signal, detecting the modulation, and producing anindication that the modulation has been detected.
 14. The method ofclaim 12 wherein selecting and modulating at least a portion of theduration of at least some of the synchronizing intervals includessinusoidally modulating at least a portion of the duration ofsubstantially all of the synchronizing intervals.
 15. The method ofclaim 14 further including transporting along the circuit secondapparatus adapted for receiving the signal, detecting the modulation,and producing an indication that the modulation has been detected. 16.The method of claim 3 wherein selecting and modulating at least aportion of the duration of at least some of the synchronizing intervalsincludes modulating the amplitude of substantially all of the durationof at least a portion of the synchronizing intervals.
 17. The method ofclaim 16 further including transporting along the circuit secondapparatus adapted for receiving the signal, detecting the modulation,and producing an indication that the modulation has been detected. 18.The method of claim 16 wherein selecting and modulating at least aportion of the duration of at least some of the synchronizing intervalsincludes sinusoidally modulating substantially all of the duration of atleast a portion of the synchronizing intervals.
 19. The method of claim18 further including transporting along the circuit second apparatusadapted for receiving the signal, detecting the modulation, andproducing an indication that the modulation has been detected.
 20. In acommunication circuit which carries at least one signal includingsynchronizing intervals by which output of the signal by first apparatuscoupled to the communication circuit is to be synchronized, a modulatorfor selecting and modulating at least a portion of the duration of atleast some of the synchronizing intervals.
 21. The apparatus of claim 20further including a detector for transportation along the circuit, thedetector adapted for receiving the signal, detecting the modulation, andproducing an indication that the modulation has been detected.
 22. Theapparatus of claim 20 wherein the modulator includes a modulator forselecting and modulating the amplitude of at least a portion of theduration of at least some of the synchronizing intervals.
 23. Theapparatus of claim 22 further including a detector for transportationalong the circuit, the detector adapted for receiving the signal,detecting the modulation, and producing an indication that themodulation has been detected.
 24. The apparatus of claim 22 wherein themodulator includes a modulator for sinusoidally modulating at least aportion of the duration of at least some of the synchronizing intervals.25. The apparatus of claim 24 further including a detector fortransportation along the circuit, the detector adapted for receiving thesignal, detecting the modulation, and producing an indication that themodulation has been detected.
 26. The apparatus of claim 24 wherein themodulator includes a modulator for generating a first sinusoid having afirst frequency, generating a second sinusoid having a second frequency,frequency modulating the first sinusoid with the second sinusoid toproduce a frequency modulated signal, and modulating at least a portionof the duration of at least some of the synchronizing intervals with thefrequency modulated signal.
 27. The apparatus of claim 26 furtherincluding a detector for transportation along the circuit, the detectoradapted for receiving the signal, detecting the modulation, andproducing an indication that the modulation has been detected.
 28. Theapparatus of claim 27 wherein the detector includes a detector fordetecting the frequency modulated signal.
 29. The apparatus of claim 27wherein the detector includes a detector for detecting the secondsinusoid.
 30. The apparatus of claim 27 wherein the detector includes adetector for detecting the first sinusoid.
 31. The apparatus of claim 22wherein the modulator includes a modulator for modulating the amplitudeof at least a portion of the duration of a relatively constant portionof the synchronizing intervals.
 32. The apparatus of claim 31 furtherincluding a detector for transportation along the circuit, the detectoradapted for receiving the signal, detecting the modulation, andproducing an indication that the modulation has been detected.
 33. Theapparatus of claim 31 wherein the modulator includes a modulator forsinusoidally modulating at least a portion of the duration ofsubstantially all of the synchronizing intervals.
 34. The apparatus ofclaim 33 further including a detector for transportation along thecircuit, the detector adapted for receiving the signal, detecting themodulation, and producing an indication that the modulation has beendetected.
 35. The apparatus of claim 22 wherein the modulator includes amodulator for modulating the amplitude of substantially all of theduration of at least a portion of the synchronizing intervals.
 36. Theapparatus of claim 35 further including a detector for transportationalong the circuit, the detector adapted for receiving the signal,detecting the modulation, and producing an indication that themodulation has been detected.
 37. The apparatus of claim 35 wherein themodulator includes a modulator for sinusoidally modulating substantiallyall of the duration of at least a portion of the synchronizingintervals.
 38. The apparatus of claim 37 further including a detectorfor transportation along the circuit, the detector adapted for receivingthe signal, detecting the modulation, and producing an indication thatthe modulation has been detected. 39-44. (cancelled).